4 litres/day is a lot of water, but it is possible that this water ingress is through a broken flexible seal around SST stern tube. I think that this tube inside the skeg is only few inches long and is connected to fiberglass stern tube inside the boat where Volvo drip-less shaft seal is attached.
During construction, they pour polyester resin into cavity of the skeg and large volume of resin contracts during curing process creating small cracks. This is not a serious issue, but when seal orotund SST stern tube is leaking, water migrates through the cracks in the resin, collects under Volvo seal then flows forward toward the engine.
When I purchased my B-31 in 2015 I noticed crack and small amount of water in resin filling the skeg. After cleaning SST tube/skeg and applying polysulfide seal I don't see any water inside.
I would not use epoxi resin to seal SST tube to the skeg. Stern tube is vibrating with rotating shaft and flexible transition is probably better and easily to repair if things go wrong. Some older Beneteaus had outside stern tubes made of fiberglass glassed to the skeg. They changed design for a reason.
Attached pics shows my boat: SST stern tube without seal, stern tube with seal and in the process of painting, crack in the skeg resin, fiberglass stern tube with Volva seal. Measuring tape shows distance of crack to Volvo seal.